A typical electron beam device comprises a hermetically sealed, i.e. vacuum tight, body inside which a cathode housing is arranged. The cathode housing comprises a filament which is heated by a current in order for electrons to be produced. The thus produced electrons are accelerated by means of a high-voltage potential and exits through an exit window of the body, typically a thin window foil supported by a support grid. Electron beam devices may be used for several purposes, such as curing of ink or adhesives, or sterilisation of volumes or surfaces. Depending on the application properties such as acceleration voltage, beam profile, shape of the electron beam device will vary. The teachings of the present invention may advantageously be applied to electron beam devices used for sterilization of a web of packaging material, since it may significantly improve the performance of electron beam devices being designed for that purpose. It is to be understood, however that it may be applied to other electron beam devices having a similar construction.
Within the field of sterilization of a web of packaging material, performance factors such as stability, durability and longevity are key issues, once the quality of the sterilization is ensured. All components mentioned and still more may be optimized in order for the electron beam device to produce the desired beam shape under any given circumstances.
The present invention relates to the context of elongate electron beam devices used for treatment of larger surface, such as webs of packaging material used for production of packaging containers. More specifically the present invention relates to improvements of such electron beam devices, in terms of ensuring adequate quality while simplifying assembly of the electron beam device.
A hermetically sealed electron beam device does not contain any vacuum pump for the purpose of keeping the vacuum. A vacuum pump is only used during manufacture of the electron beam device to create the vacuum, thereafter the electron beam device is sealed. To provide the possibility of using less highly rated pumping equipment during manufacture, and to complete and maintain the vacuum in the electron beam device, getters may be used. The term “getter” normally refers to a reactive material which chemically absorbs or binds gaseous molecules to their surface inside vacuum environments. A chemical getter provides a pumping action by a chemical reaction where a chemically active gas combines with a chemically active metal to form a solid compound. Gas molecules such as H2, CO, CO2, O2, N2, and NOx, form essentially nonreactive oxides, carbides, and nitrides. Generally, the reactions proceed by dissociative chemisorption followed by a reaction to form the resulting oxide, carbide, or nitride. Hydrogen, H2, compounds seem to be an exception, with the hydrogen being dissociatively chemisorbed then dissolved into the metal bulk. Thus, a getter is able to permanently remove small amounts of remaining gas from the evacuated space.
The use of getters further improves the performance of the electron beam device in terms of high voltage stability and rate of occurrences of arcs. Further, a longer lifetime can be expected.
There are several types of getters. A common type is the non-evaporable getters made of metals, for example zirconium or titanium.
When a non-evaporable getter is exposed to air for handling or loading into the electron beam device the material's surface will react with the surrounding gases. This means that the getter, once installed, is already enclosed in an envelope of oxides, nitrides and carbides. Hence, the getter material is “saturated”, i.e. essentially inert and will not provide an active getter-pumping surface. Activation is needed in order to start the pumping action. Activation is performed by exposing the getter to high temperature during a suitable period of time when the electron beam device has been sealed and pumped. The activation process removes the layer of surface oxides, nitrides and carbides, by their diffusion into the bulk of the getter and provides a clean metallic surface ready to react with the impinging gaseous molecules in the vacuum environment. The pumping action will continue until the getter is again “saturated”, and a new activation cycle is needed. The operational time of the getter between activation cycles depends on the amount of remaining or potentially leaking gas in the electron beam device.
The pumping action, i.e. the ability of the getter to absorb various gases, varies with the temperature of the getter. High temperatures generally generate a high pumping action.